1. Field of the Invention
The present invention relates to a novel super-thin surface-mounting-type coil component suitably mounted on a hybrid IC, such as a DC-DC converter.
2. Background of the Invention
To achieve reductions in the thickness and size of an electric device such as a portable information terminal, a popular method of mounting a so-called hybrid IC in which electronic components such as capacitors, transistors, transformers, and an IC consisting of a package of a block circuit such as a DC-DC converter, is used. Such a method involves integration of said components by surface-mounting such components on one sub-substrate, said sub-substrate being mounted on a mother substrate.
Respective electronic components of the hybrid IC are required to be reduced in size and thickness (in height). In particular, the reduction in height of a coil component (transformer, choke coil or the like) is very important and in strong demand.
For example, a surface-mounting-type transformer serving as a typical surface-mounting-type coil component has a structure in which a primary winding and a secondary winding are separately wound on a resin coil bobbin, a frame lead line being built into a side or bottom surface of the coil bobbin, both the ends of which are wounded on the frame read terminal and conductively fixed thereto by soldering, and wherein a magnetic core is fitted in the coil bobbin. However, the minimum achievable height of the coil bobbin is a about 3 mm, which is insufficient.
Therefore, as a structure which is reduced in size and height, as shown in FIG. 6, a coil bobbin is omitted, and an insulator-coated conductive line is directly wound on a vertical drum ferrite core 4, the drum ferrite core being composed of a vertical shaft consisting of a core portion 1 and flange portions 2 and 3 integrally extending from both the ends of the core portion 1 in the longitudinal direction to form a winding 5. In the surface-mounting-type transformer 10, the ends of the winding are wound on a narrow-band-shaped lead terminal 6, obtained by press-shaping a metal plate built in the flange portion 3, and soldered on the lead terminal 6.
As shown in FIG. 7, a surface-mounting-type transformer 20 has an electrode layer 8 obtained by directly printing a conductive paste or the like on the bottom surface and peripheral surface of the flange portion 3 of a vertical drum ferrite core 4' to be fixed thereto, which is arranged in place of the lead terminal 6 described above. The end of the winding 5 is conductively fixed to the electrode layer 8 through a lead groove (not shown).
As shown in FIG. 8, another type of surface-mounting-type transformer 30 has electrode layers 21 directly mounted on a core by printing on end faces 16 and 17 and bottom surfaces 18 and 19 of rectangular flange portions 12 and 13 of horizontal ferrite core 15, in which the rectangular flange portions 12 and 13 are integrally formed on both the right and left ends of a rectangular-parallelopiped core portion 11 (indicated by a broken line). The ends of a winding 22 wound on the core portion 11 are conductively fixed to the end faces 16 and 17 of the electrode layers 21 by soldering.
Presently, such a surface-mounting-type transformer 20, as shown in FIG. 7, using the vertical drum ferrite core 4' has a minimum height H=1.6 mm. In the surface-mounting-type transformer 30 using the horizontal ferrite core 15, as shown in FIG. 8, as it is very difficult to withdraw the end of the winding 22 within the dimension of the electrode layer 21, the withdrawn portion of the winding or a conductive fixed portion swelled by soldering tends to protrude beyond the outside dimensions of the transformer. Therefore, while utilizing this type of structure, it is impossible to reduce the size and height of the transformer.
In addition, when conductively fixing the end of the winding by soldering, the flange portions 12 and 13 of the conductively fixed portions occupy a large area in the electrode layer 21. In addition, heat generated by soldering is transmitted to the core portion 11, adversely affecting the coil component by heat degradation of the insulating property of the winding 22.
In a hybrid IC, when the surface-mounting-type transformer 30 using the horizontal ferrite core 15 is mounted on a substrate, a leakage magnetic flux acts on a wiring pattern on a surface opposing the mounted surface, disadvantageously decreasing the inductance value. In addition, insulation between the electrode layer 21 and the wiring pattern of the mother substrate is specially required, increasing the production time and cost of manufacture.